1. Field of the Invention
The present invention is concerned with improvements in metal enclosures intended to protect in a water-tight manner and from the air electric power resistances, in particular those intended to limit the amplitude of fault currents, these resistances being mounted inside such enclosures containing exclusively air.
2. Description of the Prior Art
It is usual to observe that industrial electric resistances are mounted in open metal protective enclosures; in fact, their more or less widespread use requires the provision of rapid evacuation of calories due to the Joule effect by judiciously using convection movements of the air.
However, it is known that such resistances, particularly those grounding resistances whose role is to limit the amplitude of fault currents during a time sufficient to allow safety means to operate must be able to withstand currents which can reach one thousand amperes and even more for a period of time varying from a few seconds to a few tens of seconds. Thus the operative elements of these resistances must be able to withstand voltages which can reach several tens of kilovolt during the circulation of the fail current. For this reason, it is a constant to provide a protective enclosure for the resistances with respect to the external medium complimentarily giving a protection to the personnel with respect to the voltage at the terminals of the active elements which results from the passage of a fault current.
The type of enclosure which is the most current does not give any protection against dust or polluting particles of natural origin such as, for example sand, or from industrial sources which most often are very fine and tend to deposit on the insulators of the securing means for the resistances inside the said enclosures; the pollution phenomena moreover is further aggravated by the presence of considerable electrostatic charges which result from the high voltages which must be withstood by the same insulators.
It is known that this type of pollution leads to the breakage and even to the deterioration of the insulators if they are not regularly cleaned, and whose extent naturally is related to the amount of pollution on the site. This type of maintenance of course is very costly, and even impossible under certain conditions of use of the networks.
In this connection, numerous solutions already have been suggested for the creation of such resistances called "maintenance free". These solutions have generally involved the use of insulators which have leak lines into the air which are considerably greater than those of the insulators which are normally necessary; the excessive size of the insulator cannot solve the problem of pollution, at the most, it spaces out the frequency of the maintenance.